The operation of expansion machines, e.g. steam turbines, and for instance with the aid of the Organic Rankine Cycle (ORC) method for the generation of electric energy using organic media, e.g. organic media having a low evaporation temperature which usually have higher evaporation pressures at same temperatures as compared with water as working medium, is known in the state of the art. ORC plants represent a realization of the Clausius Rankine Cycle in which electric energy is generated, for instance, basically through adiabatic and isobaric changes of state of a working medium. Mechanical energy is generated by the evaporation, expansion and subsequent condensation of the working medium, and is converted into electric energy. Basically, the working medium is brought to an operating pressure by a feed pump, and energy in the form of heat, which is provided by a combustion or a flow of waste heat, is supplied to the working medium in an evaporator. The working medium flows from the evaporator through a pressure pipe to an expansion machine where it is expanded to a lower pressure. Subsequently, the expanded working medium steam flows through a condenser in which a heat exchange takes place between the vaporous working medium and a cooling medium. Then, the condensed working medium is recirculated by a feed pump to the evaporator in a cycle.
One particular class of expansion machines is represented by volumetrically working expansion machines, which are also referred to as displacement expansion machines, which comprise a working chamber and work during a volume increase of this working chamber as the working medium expands. These expansion machines are realized, for instance, in the form of piston expansion machines, screw expansion machines or scroll expanders. Volumetrically working expansion machines of this type are used in particular in low performance class ORC systems (e.g. electrical power of 1 to 500 kW). As opposed to turbines, however, volumetrically working expansion machines require lubrication by means of a lubricant, in particular of the piston and the profiles (flanks) of the expansion chamber rolling upon one another, and of the rolling bearings and the gliding walls of the working chamber. Hence, it is necessary to lubricate the bearing surfaces and the contacting flanks. The use of a lubricant advantageously also leads to a sealing of the working chamber of the expansion machine, so that less steam gets lost by an overflow inside the expansion machine and the efficiency is increased. A lubrication with oil is advantageous, with oil and live steam passing the expansion machine at the same time, which necessitates a subsequent separation of the oil and the steam.
Lubrication in refrigeration engineering is easy to realize. A soluble oil is added to the working medium. At the outlet of the compression machine the oil is available in the form of finely distributed droplets in the compressed steam. The highly pressurized steam-oil spray is now passed through an oil separator where oil is separated by a cyclone and the refrigerant is discharged from the oil separator in the form of steam in the direction of the condenser. The oil is now highly pressurized and can be directly injected into the inlet area of the compression machine and transported to the bearings. The oil is entrained with the low-pressure steam, brought to a high pressure together with the steam, and can then, again, be separated in the oil separator.
The method for lubricating compressors ensued a method for the lubrication of expansion machines. In this method, oil is added to the working medium. The separation of oil and steam likewise takes place at the outlet of the expansion machine in an oil separator. As the pressure at the outlet is lower than at the inlet during the expansion, the oil has to be brought to a live steam pressure by an oil circulation pump to allow an injection of the oil into the live steam at the inlet for the lubrication of the flanks. In addition, the bearing surfaces have to be supplied with oil, too. FIG. 1 illustrates a schematic diagram of such a lubricating system according to the state of the art. A working medium is supplied by an evaporator 1 to an expansion machine 2. In the expansion machine 2 the vaporous working medium is expanded, and the released energy is converted by a generator 3 to electric energy. A lubricant, e.g. a lubricating oil, is supplied by an oil circulation pump 4 to the expansion machine 2. The lubricant serves to lubricate the bearings L and flanks F in the expansion machine. The lubricant is discharged from the expansion machine 2 together with the expanded working medium. The lubricant is contained in the expanded working medium in the form of a finely distributed oil fog and is separated from the working medium in an oil separator 5 so that the working medium is supplied from the oil separator 5 to a condenser 6 substantially free from oil. The condensed working medium is recirculated to the evaporator 1 by a feed pump 7. The recovered oil is recirculated by the oil circulation pump 4 to the expansion machine 2.
However, the lubricating system according to the state of the art has the following drawbacks. As the lubricant (lubricating oil) is separated on the low-pressure side after passing the expansion machine 2, it is required to provide the oil circulation pump 4 which, as the lubricant has to be supplied on the high-pressure side of the expansion machine 2, has to overcome the same pressure difference as the feed pump 7 that transports the working medium, which results in a great instrumentation expenditure accompanied by respective costs. Moreover, a relatively large oil separator 5 is necessary because the exhaust steam flowing out of the expansion machine 2 has a smaller density compared to the live steam supplied to the expansion machine 2, for instance, a density lower by more than one order of magnitude. This leads to a great material expenditure accompanied by respectively high costs. The large volume necessitates a large filling quantity of relatively expensive oil. Also, the separation of the lubricant from the exhaust steam of the working medium is accomplished by means of cyclone separators or baffles, always with a significant change of direction of the exhaust steam flow containing the lubricant, so that, combined with the relatively great volumes of the waste steam flow, pressure losses occur which result in a counter-pressure that acts on the expansion machine 2 and, thus, in a reduction of the efficiency of same. As the oil is present at a low pressure level, an additional pump, the oil circulation pump, has to be used.
Furthermore, the relatively large oil separator 5 has a certain inertia on account of the relatively great mass, respectively the relatively great volume of the exhaust steam, which has a disadvantageous effect when the system is started or load changes occur. Also, the lubricant injected into the live steam, the live steam generally being in a liquid state and having the temperature approximately of the exhaust steam, reduces the temperature of the live steam and the enthalpy of the live steam in an undesirable manner, and thus the achievable work.
Hence, there is a need, and the present invention is based on the object, to provide a method for lubricating volumetrically working expansion machines in which the above-mentioned problems are overcome or at least moderated.